Apparatus, systems, and methods for the reception and synchronization of asynchronous signals

ABSTRACT

An apparatus and a system, as well as a method and article, may operate to separate, in the frequency domain, a combined plurality of asynchronous data streams received at substantially the same time into a separated plurality of data streams, converting to the time domain for detection, synchronizing, and decoding.

TECHNICAL FIELD

Various embodiments described herein relate to digital communicationsgenerally.

BACKGROUND INFORMATION

In some spatial-division multiple-access (SDMA) communications systems,the SDMA uplinks may be assumed to be synchronous, such that symbolsarriving from different data streams align according to timingboundaries. However, the potential 4 microsecond symbol timing errorpermitted by some Institute of Electrical and Electronics Engineers(IEEE) 802.11 standards is longer than the 0.8 microsecond guardinterval specified in the 802.11a standard. Therefore, SDMA uplinkcommunications conducted under these conditions may actually beasynchronous, and, not synchronous as assumed. Similar problems mayoccur when SDMA radios conforming to IEEE 802.16 standards are used.

For more information regarding various IEEE 802.11 standards, pleaserefer to “IEEE Standards for Information Technology—Telecommunicationsand Information Exchange between Systems—Local and Metropolitan AreaNetwork—Specific Requirements—Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY), ISO/IEC 8802-11: 1999” andrelated amendments. For more information regarding IEEE 802.16standards, please refer to “IEEE Standard for Local and MetropolitanArea Networks—Part 16: Air Interface for Fixed Broadband Wireless AccessSystems, IEEE 802.16-2001”, as well as related amendments and standards,including “Medium Access Control Modifications and Additional PhysicalLayer Specifications for 2-11 GHz, IEEE 802.16a-2003”.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are exemplary representations of asynchronous datastreams according to some embodiments;

FIG. 2 is a block diagram of an apparatus and a system according tovarious embodiments;

FIG. 3 is a flow chart illustrating several methods according to variousembodiments; and

FIG. 4 is a block diagram of an article according to variousembodiments.

DETAILED DESCRIPTION

In some embodiments, a combined plurality of asynchronous data streamsmay be received as an SDMA communication signal in the time domain. Thereceived signal may be converted from the time domain into the frequencydomain, and then separated in the frequency domain to provide aseparated plurality of data streams. In some embodiments, a spatialdemapper operating in the frequency domain (e.g., a “frequency spatialdemapper” included in a separation module) may be used to assist inseparating the combined asynchronous data streams into the separatedplurality of data streams.

Conventional spatial demappers are designed to operate in situationswhere transmitted signals from various users arrive synchronously at thereceiving antenna(s). Thus, when signals arrive asynchrounously,receivers including conventional demappers tend to malfunction. This isbecause separating asynchronous user signals in the time domain is acomplex task, while separation in frequency domain, accomplished usingthe apparatus, systems, and methods disclosed herein, is relativelyeasy. Thus, the frequency spatial demapper included in some embodimentsmay operate in the frequency domain, and its output may be convertedback into the time domain.

For the purposes of this document, “asynchronous data streams” includeany two or more data streams that have individual, time-sequencedsegments or symbols wherein at least one of the data streams includes atleast some segments/symbols having beginning and/or ending timingboundaries that do not align with similar beginning and/or ending timingboundaries of at least some segments/symbols included in another of thedata streams. For example, the symbols may not be aligned due todifferences in allowable timing error (e.g., 4 microseconds) versusallowable guard band specifications (e.g., 0.8 microseconds), such thatlike beginning/ending boundaries are out of synchronism by a time periodgreater than the allowed guard band time period.

A “data stream” may include one or more signals including datatransmitted by a single user via one or more antennas, perhaps withdifferent phases and delays. The number of antennas used to receivemultiple data streams may not be equal to the number of data streams,and in some embodiments, may be greater than the number of data streams.For the sake of simplification, it will be assumed throughout thisdisclosure that P data streams arrive asynchronously at Q receiveantennas, and that Q is typically greater than P (although in someembodiments Q may be less than or equal to P).

The term “energy conduit” includes any type of device or apparatus thathas the capability to transmit and/or receive energy to and/or fromspace. Examples of such energy conduits include antennas, infra-redtransmitters, infra-red receivers, photo-emitters (e.g., light emittingdiodes), photo-receptors (e.g., a photocell), and charge-coupleddevices, among others.

The term “transceiver” (e.g., a device including a transmitter and areceiver) may be used in place of either “transmitter” or “receiver”throughout this document. Thus, anywhere the term transceiver is used,“transmitter” and/or “receiver” may be substituted.

FIGS. 1A and 1B are exemplary representations of asynchronous datastreams 100 according to some embodiments. In some embodiments, as mightoccur when one or more SDMA uplink data streams are formatted accordingto an 802.11 standard, such as the 802.11a standard, there can be twophases of communication: a training phase 102 and a data phase 104. Inthe training phase 102, an access point (AP) may learn thecharacteristics of spatial channels 106 associated with each stationSTA1, STA2, and STA3. In the data phase 104, the AP may poll thestations STA1, STA2, and STA3, and the stations STA1, STA2, and STA3 mayrespond by sending signals, including asynchronous data streams 100, tothe AP at substantially the same time.

At this point, the AP may be able to exploit channel state informationlearned during the training phase 102 to separate the stations'superimposed signals including the asynchronous data streams 100.

Several factors may require consideration with respect to the cause ofdata stream 100 asynchronism. Among such causes may be:

-   -   (1) a conflict between the potential segment/symbol allowed        timing error (e.g., about 4 microseconds in a IEEE 802.11        wireless local area network (WLAN)) and an allowed guard        interval (e.g., about 0.8 microseconds for IEEE 802.11a). Thus        the beginnings and ends of symbols 108 sent by the stations        STA1, STA2, and STA3 may not be aligned in time. In the        asynchronous case, one orthogonal frequency-division multiplexed        (OFDM) symbol may be interfered with by 2*(K−1) OFDM symbols        from K simultaneously operating stations STA1, STA2, and STA3,        whereas the number of interfering symbols may be only (K−1) for        the synchronous case (e.g., see FIG. 1A);    -   (2) the frequency offsets of the stations STA1, STA2, and STA3        may be different. That is, the sub-carriers of the stations        STA1, STA2, and STA3 may be offset from some selected nominal        frequency point by different amounts (e.g., see FIG. 1B); and    -   (3) the phase offsets of the stations STA1, STA2, and STA3        during the training phase 102 and data phase 104 may be        different.

In order to more completely understand the techniques disclosed herein,consider how OFDM symbols may be processed with an SDMA SISO(single-input, single-output) connection. Let {tilde over (m)}_(i), i=0,. . . , N−1 be the message to send, where N is the number of OFDM tones.In the time domain, the waveform may be expressed as:

$\begin{matrix}{{m_{n} = {\sum\limits_{i = 0}^{N - 1}{{\overset{\sim}{m}}_{i}{\mathbb{e}}^{j\; 2\;\pi\; f_{i}\frac{n}{N}T_{m}}}}},} & {{f_{i} = \frac{i}{T_{m}}},} & {{n = 0},\ldots\mspace{11mu},{N - 1.}}\end{matrix}$Here T_(m) represents the message length, such that in IEEE 802.11a andIEEE 802.11g, T_(m)=3.2 μs, and N=64.

To combat the problem of inter-symbol interference (ISI), a cyclicprefix (CP) may be added to the message in the time domain. If themessage time is T_(m), and the CP time is T_(p), the resulting symbolwaveform may then be expressed as:

$\begin{matrix}{{x_{n} = \begin{Bmatrix}{m_{n + N},} & {{- p} < n < 0} \\{m_{n},} & {0 \leq n < N}\end{Bmatrix}},} & {{{{with}\mspace{14mu} T_{p}} = {\frac{p}{N}T_{m}}},}\end{matrix}$and the symbol time=T_(m)+T_(p). For IEEE 802.11a and IEEE 802.11g,T_(p)=0.8 μs. Symbols may be sent in serial fashion, as shown in FIG.1A.

Multi-path interference may result in the superposition of multiplecopies of the message, one on top of the other, as seen at a singlereceiving antenna. This type of received signal may be expressed as:

${y_{n} = {\sum\limits_{k = 0}^{L - 1}{h_{k}x_{n - k}}}},$where L is the number of taps in channel response h. After discardingthe first p samples, y_(n) may be free of ISI if L<p. However, it maystill contain multiple copies of itself. If a Fourier transform isperformed on the samples y_(n), n=0 . . . N−1:

$\begin{matrix}{{\hat{m}}_{i} = {\frac{1}{N}\;{\sum\limits_{n = 0}^{N - 1}{y_{n}\;{\mathbb{e}}^{{- j}\; 2\;\pi\; f_{i}\frac{n}{N}T_{m}}}}}} \\{= {\frac{1}{N}\;{\sum\limits_{n = 0}^{N - 1}{\sum\limits_{k = 0}^{L - 1}{h_{k}\;{\sum\limits_{i^{\prime} = 0}^{N - 1}{{\overset{\sim}{m}}_{i^{\prime}}\;{\mathbb{e}}^{j\; 2\;\pi\; f_{i^{\prime}}\frac{n - k}{N}T_{m}}\;{\mathbb{e}}^{{- j}\; 2\;\pi\; f_{i}\frac{n}{N}T_{m}}}}}}}}} \\{= {\left( {\sum\limits_{k = 0}^{L - 1}{h_{k}\;{\mathbb{e}}^{{- j}\; 2\;\pi\; f_{i}\frac{k}{N}T_{m}}}} \right){\overset{\sim}{m}}_{i}}} \\{= {{\overset{\sim}{h}}_{i}{\overset{\sim}{m}}_{i}}}\end{matrix}$and the known channel response {tilde over (h)}_(i) may permitestimating the message {tilde over (m)}_(i) from the received signaly_(n).

Extending this method to P transmit antennas and Q receive antennas, theexpression {circumflex over (m)}_(k,i)={tilde over (h)}_(kl,i){tildeover (m)}_(l,i), k=1, . . . , Q; l=1, . . . P; i=0, . . . , N−1 may holdif the incoming signal streams include aligned (e.g., synchronized)symbols. Sampling of the streams may then be chosen so that no ISIoccurs for each stream.

In reality, an SDMA uplink message may be sent from several differentstations having clocks that are not synchronized, and alignment errorunder IEEE 802.11 may be as great as 4 microseconds (e.g., the length ofa complete OFDM symbol). In this case, for example, samples may pick uppart of the CP associated with one symbol in a first stream, and part ofa different symbol in a second asynchronous stream, introducing ISI andinvalidating the relationship {circumflex over (m)}_(k,i)={tilde over(h)}_(kl,i){tilde over (m)}_(l,i), k=1, . . . , Q; l=1, . . . P; i=0, .. . , N−1.

It should be noted that even though this relationship may not hold forthe message bits, it may be correct for general waveforms. If it isassumed that the nth sample received at the qth antenna is labeledy_(q,n), then

$y_{q,n} = {\sum\limits_{k = 0}^{L - 1}{h_{{qp},k}{x_{p,{n - k}}.}}}$Performing a Fourier transform, we have:

${\overset{\sim}{y}}_{q,i} = {{\frac{1}{N}\;{\sum\limits_{n = 0}^{N - 1}{y_{q,n}\;{\mathbb{e}}^{{- j}\; 2\;\pi\; f_{i}\frac{n}{N}T_{m}}}}} = {{\overset{\sim}{h}}_{{qp},i}{{\overset{\sim}{x}}_{p,i}.}}}$As long as the rank of matrix h_(qp) is larger than P, an inverse matrixcan exist, so we may obtain:{tilde over (x)} _(p,i) =inv({tilde over (h)} _(qp,i)){tilde over (y)}_(q,i),separating the data stream in the frequency domain. Using an inverseFourier transform, the waveform x_(p,n) may be reconstructed by:

${x_{p,n} = {\sum\limits_{n = 0}^{N - 1}{{\overset{\sim}{x}}_{p,i}\;{\mathbb{e}}^{j\; 2\;\pi\; f_{i}\frac{n}{N}T_{m}}}}},$such that SISO techniques may be applied to detect a symbol, synchronizethe sample, remove the guard band, and apply a Fourier transform toobtain the message {tilde over (m)}_(p,i).

This technique may not only operate to solve the problem of asynchronousdata streams, but can also resolve different frequency offsets betweenan AP and multiple user stations (since the operations may beindependent of frequency offset). After the asynchronous data streamsare separated, each stream reception chain can track individualfrequency offset for a designated receiver signal path. That is,compensation for frequency offset may be effected individually byprocessing each data stream according to conventional single usertechniques.

Thus, the frequency spectra shown for stations USER1, USER2, and USER3in FIG. 1B illustrate that by converting received signals into thefrequency domain, operations may become independent of frequency offsetvariations. In this case, for example, the frequency spectra are offsetby different amounts. However, since the frequency spatial demapperincluded in some embodiments is able to process the superimposedasynchronous data streams in the frequency domain, for a given frequencyf, the frequency spatial demapper can recover the originally sentsignals S₁(f), S₂(f), and S₃(f). It should be noted that the frequency fmay be located at different places with respect to different users'center frequencies, such that the difference in location reflects thedifference in frequency offsets.

FIG. 2 is a block diagram of an apparatus 220 and a system 222 accordingto various embodiments, each of which may operate in the mannerdescribed above. For example, an apparatus 220 may comprise a module 226to separate (e.g., a separation module), in the frequency domain, acombined plurality P of asynchronous data streams 200 received atsubstantially the same time into a separated plurality of data streams230. The module 226 may include a spatial demultiplexer 234 (e.g., afrequency spatial demapper, a linear demultiplexer that may implementzero-forcing and minimum mean square error (MMSE) equalizationtechniques, a nonlinear demultiplexer that may implement Vertical BellLabs Layered Space-Time (V-BLAST) architecture, etc.) to provide theseparated plurality of data streams 230.

In some embodiments, the module 226 to separate the data streams 230 mayinclude one or more fast Fourier transform (FFT) modules 238 to performan FFT on the combined plurality P of asynchronous data streams 200, aswell as one or more inverse-FFT (IFFT) modules 242 to perform an IFFT onone or more of the separated plurality of data streams 230. Theapparatus 220 may also include one or more synchronization modules 250to receive at least one of the separated plurality of data streams 230after processing by one or more of the IFFT modules 242. In someembodiments, one or more of the separated plurality of data streams 230may be formatted according to any of several standards, including anIEEE 802.11 standard and/or an IEEE 802.16 standard. Numerous variationsare possible.

For example, in some embodiments, an apparatus 220 may include one ormore FFT modules 238 to perform an FFT on the combined plurality P ofasynchronous data streams 200, a spatial demultiplexer 234 to provide aseparated plurality of data streams 230 associated with the combinedplurality P of asynchronous data streams 200, and one or more IFFTmodules 242 to perform an IFFT on at least one of the separatedplurality of data streams 230 so as to separate, in the frequencydomain, the combined plurality P of asynchronous data streams 200received at substantially the same time into the separated plurality ofdata streams 230. One or more of the separated plurality of data streams230 may include a plurality of OFDM symbols (see FIG. 1A). In someembodiments, the frequency offset associated with a first data streamincluded in the separated plurality of data streams may be differentfrom the frequency offset associated with a second data stream includedin the plurality of separated data streams (see FIG. 1B). Otherembodiments may be realized.

For example, in some embodiments, the data streams may be separated inthe frequency domain, where channels are known from training. Then thewaveforms of the data streams may be reconstructed in the time domain.Each stream may then be detected, synchronized, and samples selectedwithout ISI, perhaps using detection and synchronization modules 250.Other embodiments may be realized.

For example, a system 222 may include an apparatus 220, similar to oridentical to that described previously, as well as one or more antennas264, including a plurality Q of antennas 264 to receive the combinedplurality P of asynchronous data streams 200. The antennas 264 mayinclude may different types, including patch, slot, PIFA (planarinverted f-antenna), omnidirectional, directional, monopole, dipole,etc. The antennas 264 may form a portion of a multiple-input,multiple-output (MIMO) system.

In some embodiments, a wireless access point 268 may be coupled to theplurality Q of antennas 264. The wireless access point 268 may be usedto train one or more channels for one or more of a plurality of P usersassociated with the combined plurality P of asynchronous data streams.The system 222 may also include a processor 270 to form an SDMA Q×Pchannel matrix, known to those of skill in the art.

The asynchronous data streams 100, 200, training phase 102, data phase104, spatial channels 106, symbols 108, apparatus 220, system 222,modules 226, 238, 242, 246, 250, separated data streams 230, spatialdemultiplexer 234, antennas 264, wireless access point 268, processor270, frequency f, signals S₁(f), S₂(f), S₃(f), and stations STA1, STA2,STA3, USER1, USER2, USER3 may all be characterized as “modules” herein.Such modules may include hardware circuitry, and/or one or moreprocessors and/or memory circuits, software program modules, includingobjects and collections of objects, and/or firmware, and combinationsthereof, as desired by the architect of the apparatus 220 and the system222, and as appropriate for particular implementations of variousembodiments.

It should also be understood that the apparatus and systems of variousembodiments can be used in applications other than wireless accesspoints, and thus, various embodiments are not to be so limited. Theillustrations of an apparatus 220 and system 222 are intended to providea general understanding of the structure of various embodiments, andthey are not intended to serve as a complete description of all theelements and features of apparatus and systems that might make use ofthe structures described herein.

Applications that may include the novel apparatus and systems of variousembodiments include electronic circuitry used in high-speed computers,communication and signal processing circuitry, modems, processormodules, embedded processors, data switches, and application-specificmodules, including multilayer, multi-chip modules. Such apparatus andsystems may further be included as sub-components within a variety ofelectronic systems, such as televisions, cellular telephones, personalcomputers, personal digital assistants (PDAs), workstations, radios,video players, vehicles, and others.

Various methods may be embodied by the disclosed techniques. Forexample, in some embodiments, a wireless access point may train eachuser's Q×1 channel (e.g., the first channel for each user), one by one,for all P users, and then a frequency response may be computed for eachuser's Q×1 channel. At this point, a Q×P channel matrix, known to thoseof skill in the art, may be formed for each frequency tone. The timedomain signals of P users from Q antennas may then be converted from thetime domain into the frequency domain, segment by segment. The size of atime domain segment may be determined by the sub-carrier spacing of asignal including OFDM symbols. For example, in a signal formattedaccording to IEEE 802.11a or IEEE 802.11g, the segment may be 3.2microseconds. The segment can be longer or shorter depending on thechannel frequency variation and frequency resolution of the channeltraining.

In some embodiments, the P users' signals (or spatial channels) may beseparated in the frequency domain, tone by tone, using the previouslyformed Q×P channel matrixes and a frequency spatial demapper (e.g.,using MMSE and zero-forcing techniques). The separated P signals maythen be converted from the frequency domain into the time domain,segment by segment. At this point, the resulting P time domain signalsmay be synchronized and detected using conventional techniques.

For example, with respect to synchronization, in a conventional singleuser IEEE 802.11a system, the receiver may have knowledge of thestructures of short preambles and long preambles. It may detect thepresence of the short preambles. Then coarse synchronization may beperformed at the end of the detection to search for boundaries betweenshort preambles and between the last short preamble and the first longpreamble. A coarse frequency offset estimation may also be performed.Thus, at about the same time, the receiver may also estimate thefrequency offset between its oscillator clock and that of thetransmitter. As noted previously, various embodiments may operate notonly to separate asynchronous user signals, but also to separate usershaving different frequency offsets.

FIG. 3 is a flow chart illustrating several methods according to variousembodiments. In some embodiments of the invention, a method 311 may(optionally) begin with computing a frequency response for a number ofchannels corresponding to a plurality P of asynchronous data streams atblock 321. The method 311 may continue with receiving, at substantiallythe same time, the combined plurality P of asynchronous data streams ata plurality Q of antennas at block 325. The method 311 may also includeconverting the combined plurality P of asynchronous data streams fromthe time domain into the frequency domain at block 327.

In many embodiments, the method 321 may include separating, in thefrequency domain, the combined plurality P of asynchronous data streamsreceived at substantially the same time into a separated plurality ofdata streams at block 331. In some embodiments, the method 311 mayinclude separating the combined plurality P of asynchronous data streamsinto the separated plurality of data streams in the frequency domainusing a frequency spatial demapper at block 337. It should be noted thatit is possible for the number of the separated plurality of data streamsto correspond directly to the number of wireless channels. One or moreof the separated plurality of data streams may be formatted according toan IEEE 802.11 Standard. Separation may be performed by a wirelessaccess point.

In some embodiments, the method 311 may include converting the separatedplurality of data streams in the frequency domain into a separatedplurality of data streams in the time domain at block 341. The method311 may continue with detecting the separated data streams at block 345,and then synchronizing one or more of the separated plurality of datastreams in the time domain at block 351. In some embodiments, the method311 may include performing a coarse synchronization of at least one ofthe separated plurality of data streams after detecting the presence ofa short preamble at block 357. At about the same time, a coarsefrequency offset estimation may also be performed.

It should be noted that the methods described herein do not have to beexecuted in the order described, or in any particular order. Moreover,various activities described with respect to the methods identifiedherein can be executed in serial or parallel fashion. For the purposesof this document, the terms “information” and “data” may be usedinterchangeably. Information, including parameters, commands, operands,and other data, can be sent and received in the form of one or morecarrier waves.

Upon reading and comprehending the content of this disclosure, one ofordinary skill in the art will understand the manner in which a softwareprogram can be launched from a computer-readable medium in acomputer-based system to execute the functions defined in the softwareprogram. One of ordinary skill in the art will further understand thevarious programming languages that may be employed to create one or moresoftware programs designed to implement and perform the methodsdisclosed herein. The programs may be structured in an object-orientatedformat using an object-oriented language such as Java or C++.Alternatively, the programs can be structured in a procedure-orientatedformat using a procedural language, such as assembly or C. The softwarecomponents may communicate using any of a number of mechanismswell-known to those skilled in the art, such as application programinterfaces or inter-process communication techniques, including remoteprocedure calls. The teachings of various embodiments are not limited toany particular programming language or environment. Thus, otherembodiments may be realized, as shown in FIG. 4.

FIG. 4 is a block diagram of an article 485 according to variousembodiments, such as a computer, a memory system, a magnetic or opticaldisk, some other storage device, and/or any type of electronic device orsystem. The article 485 may comprise a processor 487 coupled to amachine-accessible medium such as a memory 489 (e.g., a memory includingan electrical, optical, or electromagnetic conductor) having associatedinformation 491 (e.g., computer program instructions, and/or otherdata), which when accessed, results in a machine (e.g., the processor487) performing such actions as separating, in the frequency domain, acombined plurality P of asynchronous data streams received atsubstantially the same time into a separated plurality of data streams.The activity of separating may be performed by a wireless access point.

Other activities may include computing a frequency response for a numberof channels corresponding to the plurality P of asynchronous datastreams. In some embodiments, further activities may include convertingthe separated plurality of data streams in the frequency domain into aseparated plurality of data streams in the time domain, as well asperforming a coarse synchronization of at least one of the separatedplurality of data streams after detecting the presence of a shortpreamble. At about the same time, a coarse frequency offset estimationmay be performed.

Implementing the apparatus, systems, and methods described herein mayresult in providing an operational SDMA uplink when transmittingstations are not synchronized, or when the accuracy of synchronizationis on the order of the CP time interval. Signals having differentfrequency and/or phase offsets may also be processed in a more reliablefashion. Less power may be required since there may be no need tosynchronize clocks with high accuracy. At the same time, WLAN throughputmay be increased by enabling transmission for multiple stations andreception with access points on a substantially simultaneous basis.

Although the inventive concept may be discussed in the exemplary contextof an IEEE 802.11x implementation (e.g., IEEE 802.11a, IEEE 802.11g,IEEE 802.11 HT, etc.), the claims are not so limited. Indeed,embodiments of the present invention may well be implemented as part ofany wireless system, including those conforming to various versions ofthe IEEE 802.16 standards, and/or using multi-carrier wirelesscommunication channels (e.g., orthogonal frequency-division multiplexing(OFDM), discrete multi-tone modulation (DMT), etc.), such as may be usedwithin, without limitation, a wireless personal area network (WPAN), awireless local area network (WLAN), a wireless metropolitan are network(WMAN), a wireless wide area network (WWAN), a cellular network, a thirdgeneration (3G) network, a fourth generation (4G) network, a universalmobile telephone system (UMTS), and the like communication systems.

The accompanying drawings that form a part hereof show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate embodiment.

1. A method comprising: computing, by a wireless access point, a channelmatrix that is representative of a channel response for each of aplurality of channels, said computing based at least in part on trainingsignals received over two or more antennas from multiple stations;receiving from multiple stations, at the wireless access point, aplurality of uplinked spatial division multiple access (SDMA) datastreams that are out of synchronism; converting the plurality of SDMAdata streams from a first time domain to a frequency domain; separating,with a spatial demapper, the plurality of SDMA data streams in thefrequency domain into a separated plurality of data streams in thefrequency domain based at least in part on the channel matrix;converting the separated plurality of data streams from the frequencydomain to a second time domain; and synchronizing the separatedplurality of data streams in the second time domain.
 2. The method ofclaim 1, wherein the receiving comprises: receiving at least some of theplurality of SDMA data streams as data streams that include a pluralityof non-aligned orthogonal frequency division multiplexed symbols.
 3. Themethod of claim 1, wherein the receiving comprises: receiving theplurality of SDMA data streams in response to a polling communication.4. The method of claim 3, wherein the polling communication comprisesmultiple polling messages overlapping in time and corresponding innumber to the multiple stations.
 5. The method of claim 1, wherein theseparating comprises: separating the plurality of SDMA data streamsusing a channel matrix.
 6. The method of claim 1, wherein the separatingcomprises: separating the plurality of SDMA data streams into aseparated plurality of data streams, wherein at least some of theseparated plurality of data streams have different frequency offsets. 7.The method of claim 1, wherein a number of the separated plurality ofdata streams correspond to a like number of wireless channels.
 8. Themethod of claim 1, wherein at least two of the plurality of uplinkedSDMA data streams are out of synchronism greater than 0.8 microseconds.9. An article comprising a memory have instructions stored thereon,wherein the instructions, when executed, cause the processor to perform:computing, by a wireless access point, a channel response for each of aplurality of channels based on training signals received over two ormore antennas from multiple stations, the computed channel responseincludes at least a channel matrix; converting a plurality of spatialdivision multiple access (SDMA) data streams from a first time domain toa frequency domain after the plurality of SDMA data streams have beenreceived as a plurality of uplinked SDMA data streams that are out ofsynchronism; separating the plurality of SDMA data streams in thefrequency domain into a separated plurality of data streams in thefrequency domain based on the channel matrix; converting the separatedplurality of data streams from the frequency domain to a second timedomain; and synchronizing the separated plurality of data streams in thesecond time domain.
 10. The article of claim 9, wherein the separatingcomprises: separating the plurality of SDMA data streams at a wirelessaccess point.
 11. The article of claim 9, wherein the plurality ofchannels correspond in number to a number of the plurality of SDMA datastreams.
 12. The article of claim 9, wherein the synchronizingcomprises: synchronizing at least one of the separated plurality of datastreams after detecting a boundary between preambles.
 13. The article ofclaim 9, wherein the instructions, when executed, cause the processor toperform: estimating a coarse frequency offset between receiver andtransmitter oscillator clocks.
 14. An apparatus, including: a separationmodule to separate a plurality of spatial division multiple access(SDMA) data streams into a plurality of separated data streams, in afrequency domain, after the plurality of SDMA data streams have beenconverted from a first time domain to the frequency domain, wherein theseparation module is configured to separate the plurality of SDMA datastreams in the frequency domain based at least in part on a channelmatrix, and wherein the plurality of SDMA data streams have beenreceived as a plurality of uplinked SDMA data streams that are out ofsynchronism by a time period greater than an allowed guard band timeperiod; and a synchronization module to synchronize the separatedplurality of data streams in a second time domain after the separatedplurality of data streams have been converted from the frequency domainto the second time domain.
 15. The apparatus of claim 14, wherein theseparation module comprises: a spatial demultiplexer to provide theseparated plurality of data streams.
 16. The apparatus of claim 14,wherein the separation module comprises: a module to perform a fastFourier transform on the plurality of SDMA data streams.
 17. Theapparatus of claim 14, wherein the separation module comprises: a moduleto perform an inverse fast Fourier transform on at least one of theseparated plurality of data streams.
 18. A system, comprising: aseparation circuit to separate a plurality of spatial division multipleaccess (SDMA) data streams into a plurality of separated data streams,in a frequency domain, after the plurality of SDMA data streams havebeen converted from a first time domain to the frequency domain, whereinthe separation circuit is configured to separate the plurality of SDMAdata streams in the frequency domain based at least in part on a channelmatrix, and wherein the plurality of SDMA data streams have beenreceived as a plurality of uplinked SDMA data streams that are out ofsynchronism by a time period greater than an allowed guard band timeperiod; a synchronization circuit to synchronize the separated pluralityof data streams in a second time domain after the separated plurality ofdata streams have been converted from the frequency domain to the secondtime domain; and a wireless access point coupled to a plurality ofantennas to receive the plurality of SDMA data streams.
 19. The systemof claim 18, wherein the channel matrix is a Q×P matrix, the systemfurther comprising; a processor to form the Q×P channel matrix, whereinthe plurality of antennas comprises Q antennas, and wherein theplurality of SDMA data streams comprises P data streams.
 20. The systemof claim 18, wherein the wireless access point is to train at least onechannel for at least some of a plurality of stations associated with theplurality of SDMA data streams.